Lignin is a phenolic heteropolymer and is the second most abundant natural terrestrial biopolymer. Unlike cellulose, which is a polysaccharide consisting of several linear chains of 1-(1-4)-linked glucose units, lignin is an amorphous and randomly branched polymer comprised of phenylpropanoid units. Lignin constitutes up to 35% of a typical woody material by mass and 50% by energy, providing strength and rigidity to the plant cell wall. Lignocellulosic biomass has the potential to produce low molecular weight compounds, with lignin being the most promising element due to its unique aromatic backbone, which is transformable to an array of value-added chemicals. For more than a century, pulp and paper industries have been pursuing research for efficient means to upgrade lignin into value-added products. A recent thrust on using lignocellulosic biomass as a feedstock for fuels and chemicals has infused new incentives for lignin valorization. However, challenges exist during lignin fractionation possesses in retaining its native structural properties. Depending on the conversion process, more structural complexity is added to the physical and chemical properties of extracted lignin. These complications have created many obstacles in the large scale application of lignin; hence a more effective and economical lignin fractionation process that creates fewer structural complexities will provide a new dimension to the lignin recovery and its consequent valorization.
The heterogeneity of lignin (both in its varied bond chemistry and its variability between plants), however, is the primary hurdle to its targeted upgrading and reuse as a feedstock for chemicals and advanced materials. Lignin is a complex hetero-biopolymer composed of three different monomers (monolignols; H, G and S) connected through as many as eight different bonding motifs. The most common bonding motif in soft (˜50%) and hardwood (˜65%) lignin is the β-O-4 linkage (Henriksson 2007, Studer, DeMartini et al. 2011). In addition to being the most abundant linkage, the β-O-4 bond is also the target of nearly all lignin pretreatment scenarios (protocols) for biomass conversion (Wilkerson, Mansfield et al. 2014). During lignin extraction and thermochemical/biochemical deconstruction processes, lignin polymers are prone to multiple chemical modifications, resulting in a heterogeneous mixture from which it is difficult to analyze and separate individual lignols (Laskar, Yang et al. 2013, Ragauskas, Beckham et al. 2014, Mottiar, Vanholme et al. 2016). Thus, the key to lignin valorization is selective depolymerization of lignin in a solvent system that is compatible with a catalyst or biocatalyst.
Lignin deploymerization can be categorized into thermochemical such as high temperature pyrolysis, hydrogenolysis, catalytic oxidation and biological using lignolytic enzymes or microbes (Laskar, Yang et al. 2013, Mu, Ben et al. 2013, De Wild, Huijgen et al. 2014, Beckham, Johnson et al. 2016). Tremendous efforts have been focused on improving the catalyst efficiency and selectivity in a thermochemical conversion process (Zakzeski, Bruijnincx et al. 2010, Chatel and Rogers 2013). However, the effectiveness and selectivity of the catalyst as developed on lignin model compounds often get compromised when applied to plant derived lignin with intrinsic structural and compositional complexity (Rinaldi, Jastrzebski et al. 2016). Biological lignin deconstruction can be performed at lower temperature and pressure than thermochemical methods while product selectivity can be potentially improved via the specificity of biocatalysts (Linger, Vardon et al. 2014, Beckham, Johnson et al. 2016).
Recent advances in the application of ionic liquids (IL) and deep eutectic solvents (DES) for biomass deconstruction and subsequent lignin extraction has given new tangent to the biomass pretreatment process. ILs are a category of molten salts at room temperature, with several desirable features, such as low-toxicity, no vapor pressure, strong polarity, high stability as compared to other organic solvents. DES is a mixture of two or more hydrogen-bond donors (HBD) and hydrogen-bond acceptors (HBA). Many DESs share similar solvent characteristics of ionic liquids (ILs). In addition, DES can be easily prepared with high purity and low cost compared with ILs. Certain DESs are capable of retaining most of the advantages from ILs while at the same time overcoming some of their limitations, which makes DES a promising candidate for multiple applications including biomass deconstruction. For instance, a cholinium chloride:lactic acid-based DES acts as a mild dual acid-base catalyst that dictates the controlled cleavage of aryl ether linkages in the phenylpropane units, leading to the delignification of biomass. In a more recent study, renewable DESs were synthesized from lignin-derived phenolic compounds for delignification of switchgrass. Although lignocellulosic biomass pretreatment using IL or DES is still in nascent stage, results from several studies indicate that IL or DES may facilitate lignin dissolution from cellulosic biomass thus improving the enzymatic hydrolysis of the resulting biomass.
Catalytic lignin valorization has been widely investigated; among them catalytic hydrogenolysis has received increasing attention. During hydrogenolysis, reductive bond cleavage takes place within lignin and/or lignin model compounds in presence of hydrogen as a reducing agent. Heterogeneous catalysts have been extensively investigated to aid the bond cleavage. However, aryl ether cleavage of lignin by hydrogenolysis with H2 requires high temperature and excessive pressure due to the low solubility of H2 in many organic solvents, posing safety concerns and operational hazards towards application of this technology. As an alternative route, catalytic transfer hydrogenolysis (CTH) has shown great promise. In CTH reaction scheme, an equivalent of H2 is transferred from a donor molecule to the acceptor molecule. Hydrogen donor molecules are often inexpensive organic alcohols capable of readily generating hydrogen molecules and the same time serving as solvents for lignin. A variety of hydrogen donating agents have been tested including formic acid, methanol, ethanol, tetralin etc., among, which isopropyl alcohol (IPA) remains a popular choice due to its relatively low cost and easy subsequent separation from the reaction mixture.
Lignin depolymerization via CTH in acids, bases and supercritical alcohols have been investigated previously. Depolymerization of lignin into monomeric phenols using formic acid, methanol, or ethanol in the presence of transition metal catalysts have been reported in several studies. An aromatic monomer yield of 6.1 wt % was obtained from CTH of concentrated acid hydrolysis lignin using Ru/C catalyst at 350° C. for 60 min and a 1:3 formic acid-to-lignin mass ratio. In a recent study, selective depolymerization of lignin to alkylphenols via CTH was reported on lignin rich residues recovered from cholinium lysinate IL pretreatment using Ru/C catalyst in IPA at 300° C. Several nanoparticles (FeB, NiB, and FeNiB) were applied for CTH of organosolv lignin in supercritical ethanol at 320° C. Results suggest that in presence of FeNiB alloy, the number-average molecular weight of lignin was reduced from 1800 Da to 317 Da, producing monomeric phenols with intact deoxygenated aliphatic side chains. CTH of alkaline lignin using Pd/C catalyst combined with metal chlorides at 260° C. was investigated and results suggest that 24% of phenolic monomers was produced using CrCl3 catalyst; however when combining with Pd/C catalyst the phenolic monomer yield was increased to 28.5%, likely attributed to the interaction of CrCl3 with oxygen electron pair to promote the crack of methoxyl groups. These studies have shed light on using platinum group noble metals and transition metal catalysts in different combinations and on various supports for assisting transfer hydrogenolysis and breakdown of various types of lignins; however the effectiveness of catalysts and CTH conditions on DES extracted lignin has not been investigated.
Biomass sorghum has received increasing attention from the biofuel research community in the last decade. As an attractive energy crop, sorghum is a promising source of biomass feedstock for biofuels because fewer inputs (e.g. nitrogen) and less water are required for growing sorghum when compared with corn production. Forage sorghum feedstock has the potential of producing 530-700 gallons of ethanol per acre as compared to the typical ethanol yield from switchgrass of 310-350 gallons per acre. The lignin fractions in sorghum contain a high abundance of ferulate and p-coumarate moieties in addition to the S/G/H lignin structural units, demonstrating a great potential for upgrading sorghum lignin to high value chemicals for various applications.
With growing interest in using IL and DES for lignin fractionation and depolymerization, it has been demonstrated that certain IL and DES can selectively cleave ether linkages and the process can generate lower and narrowly distributed molecular weight lignin. To achieve the long-term goal of developing an efficient and effective process for lignin depolymerization via IL or DES, it is important to further explore catalytic systems that depolymerize and upgrade lignin in the IL or DES medium.
The invention herein explores lignin valorization in IL and DES via catalysis and biocatalysis. As set forth herein, the methods of the invention provide valorizing lignin from lignocellulosic biomass feedstocks, to low molecular weight chemicals.